In lean-burn internal combustion engines (diesel engines, for example), a NOx purification (or removing) catalyst, which is also called as a lean NOx catalyst (LNC), may be fitted in an exhaust passage to clean the exhaust gas by trapping NOx in the exhaust gas during a lean operation, in which an air fuel ratio (referred to as A/F hereinafter) is higher than a stoichiometric A/F, and reducing the trapped NOx during a rich operation, in which the A/F is lower than the stoichiometric A/F or during stoichiometric operation, in which the A/F is equal to the stoichiometric A/F. Because there is a limit to the amount of NOx that the LNC can trap, it is not desirable to continuously conduct the lean operation for an excessive period of time. In order to release NOx trapped by the LNC, the A/F is temporarily made rich, and the released NOx is reduced by reducing agent such as HC and/or CO into N2, which is then discharged to the atmosphere (see Japanese Patent No. 2845103, for example). Such an exhaust gas control is referred to as a reduction rich control hereinafter.
The reduction rich control is conducted intermittently during the lean operation as well as in a transitional period from the lean operation to the stoichiometric operation based on an estimation of an amount of NOx trapped by the LNC (referred to as a trapped NOx amount hereinafter) during the lean operation. Regarding the reduction rich control, Japanese Patent Application Laid-Open Publication (kokai) No. 2006-207487 has disclosed estimating the amount of supply of reducing agent from an output from a linear A/F sensor (LAF sensor) provided upstream of the LNC and a space velocity of the LNC, and terminating the reduction rich control when the estimation exceeds a necessary amount of reducing agent corresponding to the trapped NOx amount.
In recent exhaust purifying devices for diesel engines, an attempt is made to dispose a three way catalyst (TWC) on an upstream side of the LNC in order to further remove harmful exhaust gas components. In the exhaust purifying devices of this type, part of the reducing agent supplied in the reduction rich control is consumed (oxidized) by the TWC and therefore, it is necessary to take this consumption into consideration when setting the amount of reducing agent to be supplied (or the duration time of the reduction rich operation). Further, because the amount of consumption of reducing agent by the TWC can change depending on the degree of deterioration of the TWC, it is necessary to measure the degree of deterioration of TWC to achieve highly precise reduction rich control.
The deterioration of TWC can be measured by additionally providing an LAF sensor (referred to as an F_LAF sensor hereinafter) on an upstream side of the TWC and comparing the output of the F_LAF with that of the LAF sensor provided on a downstream side of the TWC (this LAF sensor can be the same as the above described LAF sensor provided on the upstream side of the LNC and is referred to as an M_LAF sensor hereinafter). However, in diesel engines, the concentration of H2 and/or CH4 in the exhaust gas increases in some operational regions (in a low load and low temperature combustion region, for example), and the detection precision of the LAF sensors can be lowered when the amount of H2 and/or CH4 becomes excessively large.
This is because H2 and CH4 both have a small molecule size and thus are easy to diffuse into a diffusion rate-determining layer of the LAF sensor, and thus when there is a large amount of H2 and/or CH4 in the exhaust gas, the LAF sensor tends to output a detection value of A/F that is richer (approximately 0.5 lower, for example) than an actual A/F. When the F_LAF sensor outputs a detection value that is richer than the actual value, an erroneous judgment is made that the TWC is less deteriorated than it actually is and this can lead to an excessive supply of reducing agent to the LNC, which in turn can result in an increase in an amount of HC or CO discharged to the atmosphere and/or poor fuel consumption efficiency. Conversely, in the case where the M_LAF sensor outputs a detection value that is richer than the actual value, an erroneous judgment is made that the TWC is more deteriorated than it actually is and this can cause a NOx slip to occur and thus increase the amount of NOx discharged to the atmosphere.